1. Field of the Invention
The present invention relates generally to a deceleration-sensitive type hydraulic brake pressure control valve used for a hydraulic brake system of an automotive vehicle by which the rate of increase in hydraulic brake pressure in the outlet port thereof (this pressure is applied to the rear wheel brake cylinders) is so controlled as to become smaller than that in hydraulic brake pressure in the inlet port thereof (this pressure is directly applied to the front wheel brake cylinders), and more specifically to a deceleration-sensitive type hydraulic brake pressure control valve by which the critical hydraulic brake pressure at which the rate of increase in hydraulic brake pressure is changed is controlled according to the vehicle weight. In more detail, since the greater the vehicle weight, the higher the hydraulic brake pressure at which a constant deceleration rate is obtained, therefore the critical hydraulic brake pressure obtained when a deceleration sensor detects that the vehicle deceleration rate exceeds a predetermined level increases with increasing vehicle weight. By enclosing this critical hydraulic brake pressure within an enclosed pressure chamber, the hydraulic pressure in the outlet port is restrictively controlled according to the hydraulic pressure in the inlet port on the basis of this enclosed critical hydraulic brake pressure after the vehicle deceleration rate exceeds a predetermined value.
2. Description of the Prior Art
In a hydraulic brake system for an automotive vehicle, the front and rear vehicle wheels are braked simultaneously. However, when a vehicle is braked, because the center of gravity of vehicle body inclusive of vehicle load is shifted in the frontward direction and therefore the weight applied to the rear wheels decreases, the rear wheels tend to be locked before the front wheels are locked. In case the rear wheels are locked before the front wheels are locked, this causes a danger of skid, that is, vehicle wheels slide readily without rotating while a vehicle moves onward. For the reason described above, usually a hydraulic brake control valve is provided for the hydraulic brake system for an automotive vehicle. This control valve serves to reduce the rate of increase in hydraulic brake pressure applied to the rear wheel brake cylinders in comparison with that in hydraulic brake pressure applied to the front wheel brake cylinders.
As the hydraulic brake pressure control valve of this type, a proportioning valve or a limiting valve is well known. In these valves, only a spring-actuated valve is disposed in the outlet port thereof for simply reducing the hydraulic brake pressure supplied from the outlet port, so that there exists no critical hydraulic brake pressure at which the rate of increase in hydraulic pressure is changed; that is, the brake pressure is constant irrespective of the vehicle weight. In more detail, the ratio of front wheel cylinder hydraulic brake pressure P.sub.f to rear wheel cylinder hydraulic brake pressure P.sub.r is constant regardless of the vehicle weight. However, the ideal front-and-rear wheel cylinder hydraulic brake pressure ratio (P.sub.f :P.sub.r) must be controlled according to the vehicle weight, in order to simultaneously lock the front and rear wheels. Therefore, it is necessary to increase this critical hydraulic brake pressure in proportion to an increase in vehicle weight. From this standpoint, the above-mentioned two valves are not suitable for large vehicles such as trucks or buses, because the weight of these vehicles changes markedly between where the vehicle is loaded and where the vehicle is unloaded.
To overcome the above-mentioned problem, there has been proposed a deceleration-sensitive type hydraulic brake pressure control valve. In this control valve, the deceleration rate of the vehicle is sensed; the hydraulic brake pressure obtained when the sensed deceleration rate exceeds a predetermined level is enclosed within an enclosed pressure chamber (in this case, the enclosed hydraulic brake pressure increases with increasing vehicle weight, because the more the vehicle weight, the more the hydraulic brake pressure even if the deceleration rate is constant); a so-called piston-type gravity valve (G valve) is actuated with the one end of the valve disposed within the enclosed pressure chamber, in order that the critical hydraulic brake pressure is increased with increasing vehicle weight.
In this prior-art deceleration-sensitive type hydraulic brake pressure control valve, however, if the brake pedal is abruptly depressed deeply when the vehicle is unloaded, since the response speed of the piston-type G-valve in the valve-closing direction is not high, there exists a problem in that the critical enclosed hydraulic brake pressure is increased excessively; that is, the critical brake pressure does not well correspond to the vehicle weight. To overcome this problem, an orifice is provided in the hydraulic fluid passage communicating between the master cylinder and the control valve in order to prevent the enclosed hydraulic brake pressure from rising excessively when the vehicle is braked abruptly. However, this action raises another problem in that if the brake pedal is abruptly depressed deeply when the vehicle is loaded, the enclosed hydraulic brake pressure is conversely decreased excessively; that is, the critical brake pressure does not also well correspond to the vehicle weight. This is because the presence of the orifice prevents a quick rise in the enclosed hydraulic brake pressure and therefore the piston-type G-valve is closed before the hydraulic brake pressure reaches the ideal critical pressure.
In addition to the above-mentioned drawback, in this prior-art deceleration-sensitive type hydraulic brake pressure control valve, since the hydraulic brake pressure supplied from the master cylinder is directly exposed to the inertia-sensitive gravity valve (G valve), when an emergency brake force is applied to the vehicle, a high hydraulic brake pressure directly strikes against the inertia-sensitive G-valve, thus resulting in a problem in that the stability of the G-valve responsive to deceleration rate is deteriorated. In other words, there exists a problem in that the rear wheels are locked earlier than the front wheels or the brake force is not sufficient.
A more detailed description of the prior-art deceleration-sensitive type hydraulic brake pressure control valve for an automotive vehicle will be made with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.